For early detection and blood pressure management of lifestyle related diseases caused by hypertension, a sphygmomanometer is being widely used. In recent years, new clinical states of the hypertension have become apparent with the wide spread use of the sphygmomanometer.
For example, a clinical state of morning hypertension shows a daily fluctuation pattern where a blood pressure is usually normal but is specifically high for one to two hours after waking up, and is known to become a strong risk factor of brain and cardiac disease through research. A clinical state of nocturnal hypertension shows a pattern where the blood pressure does not lower or the blood pressure rises during sleep compared to the daytime, and is known to become a strong risk factor of brain and cardiac disease and is deeply related to a sudden death.
Furthermore, the nocturnal hypertension is known to be strongly associated with a sleep apnea syndrome in which breathing stops during sleep. When breathing stops during sleep, a sympathetic nerve activity is increased by hypoxemia or an arousal reaction, and the blood pressure becomes high at night. Thus, measuring the blood pressure during sleep is effective in diagnosis and treatment of hypertension.
As a method of measuring the blood pressure during sleep, there is adopted a method of monitoring a value of an oxygen saturation level, and selectively activating the blood pressure measurement when the value satisfies a predetermined condition, and a method of selectively activating the blood pressure measurement when satisfying a predetermined lowering condition by apnea disclosed in, for example, Japanese Unexamined Patent Publication No. 62-155829 (hereinafter referred to as Patent Document 1). The blood pressure at the time of a physiologic change such as hypoxia can thus be grasped and comparison can be made with the blood pressure at normal time.    [Patent Document 1] Japanese Unexamined Patent Publication No. 62-155829